A vacuum heat insulating material produced by packing a core material having shape retention properties with a gas barrier film and evacuating the inside of the film exhibits good heat insulating properties by evacuating gas remaining in the core material. Thus, a material having compressive strength to withstand the evacuation and being capable of forming a vacuum insulation space is used for the core material.
Conventionally known examples of the core material for a vacuum heat insulating material include (1) a material obtained by solidifying inorganic fine particles such as silica powder or perlite powder; (2) open cell polyurethane foam; and (3) a material obtained by solidifying glass fibers. The material (1) is made of super fine particles and thus causes a problem of dust particles in the production and disposal processes. The material (2) is easy to handle but difficult to recycle. Also, the material (2) needs two to three hours' predrying at 120° C. as a pretreatment prior to vacuum packing because the resin contains a minor amount of volatile matters such as water and carbon dioxide gas. The material (3) has a problem in handling caused by scattering of glass fibers as in the case with inorganic powders.
Recently, a polystyrene open cell foam for a vacuum heat insulating material has been disclosed in PC(WO)H11-504362. Such a polystyrene resin foam, which is light in weight and easy to handle, is excellent as a core material for a vacuum heat insulating material. However, a vacuum heat insulating material obtained using the foam for a core has a great change in its heat insulating properties with time and does not have satisfactory quality.
An object of the present invention is to provide an open cell polystyrene resin foam suitable as a core material for a vacuum heat insulating material and a method for producing the foam. Another object of the present invention is to provide a core material for a vacuum heat insulating material comprising the foam and a vacuum heat insulating material produced using the core material.